The invention relates to motion tracking.
Motion tracking can use a variety of measurement modes, including inertial and acoustic measurement modes, to determine the location and orientation of a body.
Inertial motion tracking is based on measuring linear acceleration and angular velocity about a set of typically orthogonal axes. In one approach, multiple spinning gyroscopes generate forces proportional to the rates at which their spinning axes rotate in response to rotation of a tracked body to which the gyroscopes are attached. These forces are measured and used to estimate angular velocity of the body. Micro-machined vibrating elements and optical waveguide based devices may be used in place of gyroscopes.
Accelerometers generate signals proportional to forces which result from linear acceleration. In an inertial tracking system, the angular velocity and acceleration signals are integrated to determine linear velocity, linear displacement, and total angles of rotation.
As the signals generated by gyroscopic devices are noisy, the integration process results in accumulation of noise components, which is generally known as “drift”. Miniaturized and low cost gyroscopic devices typically exhibit greater error. Drift rates can be as high as several degrees per second for a body at rest, and several degrees for every rotation of the body by 90 degrees. Errors in orientation estimates also affect location estimation as the estimated orientation of the body is used to transform acceleration measurements into the fixed reference frame of the environment prior to their integration. Inaccuracy in this transformation can result in gravity appearing as a bias to resulting horizontal acceleration measurements.
One way to correct drift is to use additional sensors, such as inclinometers and a compass to occasionally or continually correct the drift of the integrated inertial measurements. For instance, U.S. Pat. No. 5,645,077, issued to Eric M. Foxlin on Jul. 8, 1997, discloses such an approach. This patent in incorporated herein by reference.
Another approach to motion tracking uses acoustic waves to measure distance between one or more points on a body and fixed reference points in the environment. In one arrangement, termed an “outside-in” arrangement, a set of acoustic emitters at the fixed points on the body emit pulses that are received by a set of microphones at the fixed reference points in the environment. The time of flight from an emitter to a microphone is proportional to an estimate of the distance between the emitter and the microphone (i.e., the range). The range estimates from the emitters to the respective microphones are used to triangulate the location of the emitters. The locations of multiple emitters on the body are combined to estimate the orientation of the body.
Other measurement modes, such as optical tracking of light sources on a body, can also be used to track motion of the body.